1. Field of the Invention
The invention relates generally to shock and vibration isolators, and more particularly to a clutch/damper for isolating the oscillatory reactive forces of a driven mass from the driving motor.
2. Description of the Prior Art
There are many applications in which energy from a drive motor is applied through a gear train to drive a load mass and wherein, when subjected to environmental vibrations, the mass exhibits reactive forces which are coupled through the gear train and adversely affect the driving motor. When the system resonates, reactive torques are induced in the driving motor which may cause the motor to stall or actually be driven backwards. High reliability stepper motors have been commonly applied for drive systems because they are relatively inexpensive and the drive system electronics are less complex than for alternating current or DC current variable speed motors. It has been found, however, that for a given motor size, less drive torque is available than with corresponding alternating current or direct current motors, and the speed-torque characteristics of the stepper motors are not as advantageous. In order to overcome the presence of the reactive torques, the prior art applied a frictional damper in parallel with the drive system or simply utilized a grossly increased in size drive motor to over-power the feedback torque. These solutions have the disadvantage that the use of larger motors, which are in themselves a component of the suspended mass, increases the driven mass, thereby aggravating the problem, and a parallel friction damper requires additional motor torque to overcome the friction.
FIG. 1 exemplifies the problem as applied to a radar apparatus. A base 10 houses the electronics for driving an azimuth motor and gear train 18 and elevation motor and gear train 22, and other electronics. The base 10 is supported against vertical wall 14 by vibration and shock isolators 12. An azimuth support structure 16 has mounted thereon the azimuth motor and gear train 18 and is provided with a pivot 20 for supporting an R.F. section 26 of a radar receiver/transmitter. The R.F. section 26 pivots in elevation on elevation pivot 24 about an axis x--x so as to permit scanning of the receiver/transmitter platform 28 in a vertical direction with simultaneous pivoting in the azimuth direction about azimuth pivot 20.
Antenna 27 is also supported by the platform 28. While the combination of the antenna and receiver/transmitter into one assembly eliminates the need to run waveguides from the antenna to the receiver/transmitter, difficult vibration problems result in the antenna drive system. It may be seen that the R.F. generating and receiving components are located on the elevation gimbal of the platform 28. This creates an additional mass which must be energized by the drive system and, in practical applications, these components are very sensitive to vibration and therefore must be isolated from the vibration environment. Since it is not feasible to isolate just the R.F. components in this design, isolation has been provided by the mounts 12 between the base and electronics 10 and the aircraft mounting interface 14. However, since the center of mass of the system lies at a point 30 which is not co-planar with the vibration isolators, the unit tends to oscillate about a point below the isolator mounting plane under vibration. This oscillation generates substantial accelerations in the R.F. section 26, which are then transmitted as reaction torques through the drive systems to the respective azimuth and elevation motors.
The present invention overcomes the disadvantages of the prior art by decoupling the load mass from the driving motors when the reaction torques exceed a predetermined level, and by allowing the load mass to be driven by the drive motors when the reaction torques are less than the predetermined level.